This invention applies a two-layer parison to a core rod in successive injection molds and then transfers the two-layer parison to a blowing mold. The practical difficulty, in the prior art, to the injecting of two layers of plastic on a core rod before blowing has been that the second layer damages the first layer.
The usual way of applying a parison to a core rod, in an injection mold, is to inject the plastic material into the mold cavity at the end of the cavity remote from the neck of the core rod. Thus the plastic material is applied to the end of the core rod which is furthest from any contact with the metal of the mold; and the injected material highly heats the end of the core rod since the only way in which heat can flow from the end of the core rod is lengthwise toward the neck and through the relatively restricted cross-section of metal.
When the core rod was transferred to a second injection mold, and the injection operation repeated, the hot plastic material would contact with the hottest part of the plastic of the first layer and would wash some or all of this material off the tip of the core rod leaving no first layer on the tip portion of the core rod, or at least very little material of the first layer. This produced an unsatisfactory parison; but if sufficient time were allowed to lapse for the first layer to cool to a temperature where it would not be washed off, then the time cycle of the molding machine was increased to such an extent as to be impractical.
This invention obtains a second layer of plastic on the parison without requiring extra cooling time for the first layer. The new result is obtained by applying the second layer by neck gating. The first layer of plastic cools as it spreads along the core rod toward the neck end of the cavity. This cooling results from the fact that the plastic material is in contact with the wall of the cavity and is also in contact with portions of the core rod which are progressively closer to the neck which is held by the mold. The metal of the cavity wall and of the core rod neck, and the metal of the mold which grips the core rod neck all provide heat sinks into which heat flows from the plastic that forms the first layer of the parison. Thus the temperature of the first layer of the parison is progressively lower toward the neck of the core rod.
This invention injects the plastic for the second layer of the parison at the neck end of the second injection mold cavity. Thus the hot plastic for the second layer comes into contact first with the coolest part of the first layer and this material of the first layer is cool enough to withstand the flow of the second layer material without being washed off the core rod or even deformed by the flow of the new material. The material for the second layer is cooled by contact with the lower temperature plastic at the first layer and as the second layer material flows toward the other end of the core rod, the flowing material is cooled by contact with the wall of the cavity of the second injection mold. Thus it reaches the tip end of the core rod at a lower temperature than its injection temperature and at a time when the first layer on the tip of the core rod has had more opportunity to cool.
By making a parison in accordance with this invention three advantages are obtained. The invention eliminates the mark from the bottom of the blown article because the runner mark left on the tip end of the first layer is covered by the second layer. The invention shortens the cycle because it is not necessary to wait for the tip of the first material to cool; and it makes it possible to mold on top of a much thinner primary layer.
Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.